1. Field of Invention
The present invention relates to an apparatus and method for testing the performance of an RF receiver; and more particularly, an apparatus and method for verifying the performance of a digital receiver.
While the invention is subject to a wide range of applications, it is especially suited for testing the performance of a mobile terminal for digital satellite communication having a quadrature phase shift keyed digital receiver capable of receiving voice and data transmissions, and is described in that connection.
2. Description of Related Art
In a typical satellite communication system, incoming signals from a conventional wired telephone are conducted from the public switch telephone network to a satellite ground station, which in turn transmits RF signals to the satellite for retransmission to a mobile terminal unit. Presently, a satellite system that covers large geographical areas typically uses several satellites that follow different paths at low or medium altitudes so that at least one satellite is at all times covering the desired geographical area. From the standpoint of receiving signals, the low and medium altitude satellites have the advantage of being able to transmit a signal that reaches a mobile terminal unit at the earth's surface with a relatively large amplitude and without appreciable fading. It has been proposed, however, to provide a satellite communications network that utilizes a high altitude geosynchronous satellite which is capable of covering an area corresponding to a substantial portion of the North American Continent, so that a total of approximately six satellite beams will cover the entire Continent from Alaska to Mexico. The satellite for such a network will be approximately 22,600 miles above the equator and will be designed to operate in the L-band of RF frequencies. Energy traveling this great distance undergoes huge attenuation such that the power flux density incident at the antenna of the mobile unit is approximately 10.sub.14 watts per square meter. This grossly attenuated signal is further degraded by background noise, and other satellite channel impairments such as Rician Fading, gaussian noise, phase noise, and co-channel and adjacent channel interference.
Mobile terminal units capable of receiving these highly attenuated and impaired signals have been developed. One type of such mobile terminal includes a satellite receiver that has analog, digital and software subsections. In order to ensure that these receivers are capable of working as intended, it is of course necessary to test them under various operating conditions prior to leaving the factory. The effectiveness of the receiver should be able to be determined by merely receiving a communication over the satellite system, listening to the telephone handset, receiving a fax from the fax machine, or displaying an input from a personal computer connected to the mobile terminal. However, in order to provide defect-free mobile terminals it is necessary to test the receiving portions of such terminals under all atmospheric conditions to which the satellite system would encounter. Although, the receivers could be tested at times by using signals in the actual operating environment, it is practically impossible to subject each receiver to all of the interferences and impairments of the actual operating conditions.
In light of the foregoing, there is a need for an apparatus and method of verifying the performance of a satellite digital receiver in response to signals that are subject to all of the possible interferences and impairments of a satellite system in actual operation.